The present invention relates to a rotary voice coil motor (VCM) with a flat coil for use in driving a magnetic head or the like and, more particularly, to a magnetic circuit for the rotary VCM.
A magnetic disk apparatus has a magnetic head driving mechanism for positioning a magnetic head over a prescribed cylinder of a magnetic disk. Such a magnetic head driving mechanism includes a suitable driving motor selected according to the characteristics of the particular type of the magnetic disk apparatus.
In a hard disk drive to which the access speed and the positioning accuracy of the magnetic head are especially important, a high-torque VCM is used as the driving motor. Whereas the VCM may be either a linear VCM or a rotary VCM, the latter is suitable because of a reduction in drive mechanism size. Thus, in recent years, rotary VCMs have been used in many hard disk drives for the head driving mechanism.
The head driving mechanism using the rotary VCM has a moving coil whose motion is transmitted to the magnetic head and permanent magnets provided opposite to the moving coil. A magnetic flux generated by a driving current to the moving coil and another magnetic flux generated by the permanent magnet interact to create an attractive force or a repulsive force and thereby to provide a driving force for the positioning of the magnetic head.
It is difficult to construct a rotary VCM so as to generate a constant torque within its entire moving range because of a distribution of flux density of the permanent magnets. The rotary VCM is usually used for servo control. The above-mentioned head driving mechanism, for instance, is composed so as to feed back a signal indicating a positional relationship between the head and the disk to drive the rotary VCM. Therefore, the accuracy of head positioning is not substantially affected by torque fluctuations caused by the distribution of the flux density.
However, in a servo control system where a time constant of the system is determined according to an average torque level, the time constant fails to be optimal in a position where the torque deviates from its average, and positioning to a target position takes a long time. Thus, the unevenness of the torque results in a long delay in head positioning.